How to disable picture-in-picture in Firefox?
Solution 1:
To hide the button just right click it and click hide. You won't see it anymore.
Solution 2:
I see the same thing. In about:config, setting media.videocontrols.picture-in-picture.enabled to false doesn't do much, but setting media.videocontrols.picture-in-picture.video-toggle.enabled to false as well seems to do the job. Tested in Firefox 71 beta.